Gas-measuring devices are used for industrial gas measurement and protect persons who are located in industrial areas or in buildings in which gases that are hazardous for health, be they process gases or waste gases, may be present, from risk to health and life.
Industrial gases are monitored by gas-measuring devices in industrial environments, for example, in the chemical or petrochemical industry, to determine whether these gases are associated with hazards based on explosive or toxic properties. Gas-measuring apparatus or gas-measuring devices used stationarily are used in many cases. Infrared-optical measuring sensors, electrochemical sensors, catalytic sensors or semiconductor gas sensors are usually used as sensors in such gas-measuring devices.
It is important for the reliability of the gas-measuring devices as well as of the alarms and warnings generated thereby, that the gas-measuring devices be fully able to function during the operation and that malfunctions be reliably detected.
A thin-layer semiconductor gas sensor is described in U.S. Pat. No. 4,338,281A. The thin-layer semiconductor gas sensor has an integrated heating element. It is a metal oxide semiconductor, in which the electrical resistance of the metal oxide semiconductor layer has a value that is dependent as a function of the concentration of the gas to be detected. This change in resistance can be measured as an indicator of the presence of the gas to be detected by a suitable electronic analyzing unit associated with the thin-layer semiconductor sensor.
A catalytically operating detector circuit for combustible gases is described in U.S. Pat. No. 4,854,155 A. The detectors for the combustible gases are configured as sensor resistor elements with a catalytic coating. Detectors for combustible gases are used to detect the presence of combustible gases that may occur, for example, in mines or industrial plants.
U.S. Pat. No. 5,565,075 A describes an electrochemical gas sensor for detecting nitrogen monoxide. In a housing filled with an electrolyte, the sensor has a working electrode, a reference electrode and a counterelectrode.
Such an electrochemical gas sensor can preferably be used for medical applications, because it has a low cross sensitivity to other gases usually used in this environment.
US 2005/0247878 A1 describes an infrared gas sensor. Two infrared radiation detectors arranged next to each other are arranged in a housing on one side of the housing, while a radiation source, which emits infrared radiation, is arranged on the other side. Gas to be analyzed is introduced into the beam path from a measuring environment.
The measurement effect is based on the fact that the light emitted by the radiation source is attenuated as a function of the gas species in the beam path. One of the two detectors is operated as a reference detector, while the other detector is operated as a measuring detector. The concentration of the gas introduced into the beam path is determined from the ratio of the signals of the measuring detector and of the reference detector.
In many application situations, such gas sensors are combined or developed into stationary gas-measuring devices. Stationary gas-measuring devices are often and usually distributed in industrial plants as a plurality of sensor units for gas measurement over a plurality of rooms or larger areas.
U.S. Pat. No. 6,182,497 B1 describes a gas-measuring system, which is configured to connect a plurality of sensors to a central analysis unit. The sensors may be connected, for example, via a universal, serial bus.
A gas sensor with an adapter is known from U.S. Pat. No. 7,406,854 B2. The adapter is configured for connecting a flexible tube. It is possible via this flexible tube to deliver gases from a remotely located measuring site or a measured gas or a calibrating gas to the gas sensor for testing the ability of the gas sensor to function. Measured gas or calibrating gas may be delivered, for example, by means of a feed pump.
WO 1999/17110 A1 as well as U.S. Pat. No. 7,645,367 B2 disclose gas-measuring systems comprising a gas sensor and a gas generator. Such combinations of gas generators and gas sensors make it possible to test the measuring properties of the gas sensors, especially to determine whether the gas sensor responds sensitively to the admission of a predefined measured gas concentration.
A device for testing a gas sensor is known, for example, from DE 20 2006 020 536 U1. A gas generator, which is suitable for generating ethane, is described there. The gas generator is intended for testing the gas sensor and is configured to dispense a certain quantity of a test gas to/into the gas sensor, and a resulting change or reaction of the output signal of the gas sensor represents an indication of the ability of the gas sensor to function.
The U.S. Occupational Safety Administration (OSHA) has recommendations for function tests with so-called “bump tests,” in which a regular testing of gas sensors is to be performed by means of suitable adapters and a suitable test gas.
U.S. Pat. No. 7,406,854 B2 describes an adapter for testing or calibrating an electrochemical gas sensor. The adapter can preferably be placed on the gas sensor with a Velcro fastener and can be removed from same after completion of the testing or calibration.
There is a need for a regular testing of the ability of the gas sensors to function especially for already existing plants or installations of gas-measuring systems. In particular, there is a need for the testing of the gas sensors to be able to be performed without removal or disassembly of the gas sensors being necessary at the given measuring site in the plant. The testing of gas sensors in already existing gas-measuring systems shall be made possible without complicated manual operations performed by maintenance workers being necessary, for example, in case of the use of the adapter according to U.S. Pat. No. 7,406,854 B2, because it would otherwise be necessary for the maintenance workers in a larger industrial plant to mount the adapter for the subsequent testing of the sensors and then to remove the adapter manually when the gas sensor is put into operation. Furthermore, there is need for determining that gas sensors that deliver a gas concentration measured value as an output signal, from which the absence of harmful gases, i.e., a trouble-free situation, can be inferred, are indeed in an effective gas exchange with the measuring environment at the measuring site with the sensor-active elements. Consequently, there is a need for avoiding misinterpretations, especially of constant, noncritical gas concentration measured values or other constant output signals of the gas sensor.